The subject matter is generally directed to the field of the etching of hourglass shaped mesas in crystalline material. This is a standard process in the fabrication of buried heterostructure laser diodes and amplifiers. It is important to the device performance that the mesa width be precisely controlled. The invention is specifically directed to a visual in-process monitor or aid to the progress of the etching. This process might also be applied to the fabrication of other integrated optical components such as waveguides and couplers.